The extensor carpi ulnaris muscle runs from the thumb side of the side of the elbow to the pinky side of the wrist.

To understand what is going on at the site of your tendon, check out my post #10 in this thread. http://tt.tennis-warehouse.com/showthread.php?t=409393 [Although the post is on tennis elbow, the same thing is likely happening in your ECU tendon, just at the wrist end instead of the elbow end. So the same explanations and advice, including rest and rehab exercises would also apply to you.]

A note on the Flexbar red: You can see from the illustration of ECU muscle above that it spirals down around the forearm before inserting at the wrist. That's why doing the twisting motion in the Tyler Twist exercise would seem to be particularly good to strengthen the muscle.
Thera-Band FlexBar Video Demonstration http://www.youtube.com/watch?v=zB3TVb8a5mk

While a stiff frame and strings is known to contribute to tennis elbow, it could also contribute to your ECU tendonitis. What type of racquet and strings do you play with?

Also, if you have any doubt about your technique, having a pro check out not only your forehand, but also your backhand and serve would be a good idea. (The serve in particular involves ulnar deviation of the hand, and could be a main source of your problem.)